Electronically controlled time delay system



8, 1942, D. E. RICHARDSON ET A1,

ELECTRONICALLY CONTROLLED TIME DELAY SYSTEM Filed Jan. 12, 1940 5 J W m: w s f 6 r Z M $5 1 M a 73 a a 0 Z 3' a L mm P 2 (V F m W4 m 5 mm a m m w .z/Ulull ag 9 m Z Wu 7 z 6/ IL Patented Dec. 8, 1942 msc'rnomgmr comaouzn 'rma m srs'rnsr I Donald E. Richardson and Robert a. Yates, can cage, 11]., assignors to Drying Systems, Inc, Chicago, 111., a corporation of Illinois Application January iz, mo, Serial No. 813,639

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This invention relates to control devices andparticularly to electrically operated safety controls operable to afford a time delay of selected duration which maybe interposed, for example,

' when other apparatus is to be set in operation, to

thereby insure proper conditioning of such apparatus for operation.

There are numerous instances where it has been found desirable or necessary to aflord a comparatively long interval between the time when machinery or other equipment is conditioned for operation and the time when it is rendered fully operative. An example of such an instance is in connection with commercial heating systems including baking or drying chambers. starting up the system but before setting the burners or other heating units in operation, to interpose a time interval on the order oi, say, several minutes to a half hour, or longer, for the purpose of enabling the ventilating equipment included in the apparatus such as circulating fans and the like, to clear the combustion chambers and allied parts of the system-of any-combustible gases or vapors that might be present therein. Thereafter the heating units may be rendered operative, which is to say, the burners may be ignited, without the danger of explosions or like hazards occurring.

Heretofore it has been proposed to afford time delays of the magnitude indicated above by mechanical or electromechanical means, such as synchronous electric clockwork mechanism, of

' rather intricate construction and having a large number of moving parts.- It has been found, however, that such mechanical devices require frequent servicing, and moreover, where they are used in conjunction with electrical control ap paratus of a character entailing only non-moving parts and electrical connections therefor, the

service costs are relatively large due to the practical necessity of separately servicing the two types of control equipment. A further disadvantage of mechanical timers is that they are' ,somewhat inflexible in their construction, as

In such systems it is customary, after to have timing equipment that is to be used with such control equipment also operated electrically.

Prior types of electrical timing devices that have been free of moving parts have only been useful to afford comparatively short delay intervals, that is, time delay of two minutes or less. Such devices have not heretofore been capable of ailording longer time delays with sunlcient reliability and accuracy, and hence, despite the above set forth and other advantages, they have not been employed, for example, in safety con trol apparatus designed for use in commercial heating systems, where much longer time delays are required, as has been explained hereinabove.

The charging or discharging time of an electrical condenser may advantageously be employed to afford a time delay and this may be done without the use of moving parts, and it is an object of our invention to do so in a novel manner.

It is well known that when a condenser is being charged or discharged, there is initially almost a straight-line variation of voltage with re:

spect to time, but when the'charge on the condenser commences to approach a steady-state value after expiration of a relatively long time interval, the variation in voltage with respect to time tapers off to smaller and smaller increments, and the time required to attain a given voltage level becomes quite uncertain.' For this reason, it is necessary, in a condenser-controlled .timing device, to insure that the voltage variation during the selected delay interval remains within the substantially linear range of the charging or discharging characteristic (variation of voltage with'respect to time) of the condenser. In devices of this type the varying ,voltage or the condenser is usually employed to vary the potentialof the control grid of an electron tube, and when this potential reaches a predetermined level, thetube passes plate current to'thereby operate a means, such as arelay, in the plate circuit of the tube. The time required for the condenser voltage to reach this critical level is the measure of the amount of delay afforded.

In all such prior devices the arrangement of the circuit elements has been such that the charging or discharging characteristic of the condenser has provided a range of varying grid voltage values lying entirely to one side of the cathode potential of the tube. Hence, because it was undesirable to work along the end of the characteristic, the critical gridvoltage oi the tube had to be adjusted to a value relatively far removed from the zero or cathode level. We have found that under these circumstances, the perupon completion of the timing operation.

' time of a condenser'to control the extent formance of the timing device is quite unsatisfactory where time delays of several minutes to a half hour or longer are required. Among other things, the timing is seriously affected by fluctuations in the value of the supply voltage when the critical or firing voltage of the tube is not at or near zero as compared with the initial grid voltage applied thereto at the initiation of the timing operation. For instance, a drop of a few volts in the line voltage has been observed to affect the accuracy of such prior devices by five or six minutes over a period of about a quarter of an hour, and in a commercial burner installation this could easily prove disadvantageous or even dangerous.

A further disadvantage, particularly in the case of those devices which rely upon the discharging time of a condenser, is that leakage and other common defects in the condenser produce a very unsatisfactory condition by unduly increasing the rate of discharge of the condenser and thereby shortening the delay period. This, of course, could easly lead to a dangerous condition in a :burner installation, for example, by shortening the time ofthe scavenging period and prematurely initiating operation of the burners when there may still be combustible gases present in the system which have not yet been cleared out by the ventilating equipment.

In view of the foregoing, we rely upon the charging time of a condenser rather than upon its discharging time, whereby the device remains in its normal inoperative condition if the condenser has excess leakage. To insure accuracy of timing substantially independently of the fluctuations in line voltage which would ordinarily be encountered in practice, we have so arranged the circuit that the critical or firing voltage of the tube is at or near zero in comparison with the voltage initially applied on the control grid;- and the charging characteristic of the condenser is such that it tends to sweep through a range of values which includes this critical voltage at a substantially linear portion thereof, thus eliminating to a great extent the uncertainty in timing during relatively, long intervals, which formerly discouraged the use of electrical timers for this purpose.

Another disadvant ge which has existed in priorelectrical timing devices employing condensers is that no provision has been made to automatically return'the condenser to its normal condition upon failure of the supply voltage or Fallure to normalize the condenser in either of these events results in the condenser being left in a partially charged or discharged condition, and when the apparatus is subsequently recycled, the

condenser does not commenceto vary its voltage from the proper starting point and hence the timing is adversely affected with the detrimental effects indicated hereinabove.

Thus, a primary object of the present invention is to afford an accurate and reliable time delay,

of relatively long duration by the use of novel apparatus of simple and economical construction and without the use of moving parts.

Another object is to afl'ord a high degree of flexibility in the arrangement of the component parts of the timing means and to enable the duration of the time delay afl'orded by such. means to be selected within wide limits.

Further objects are to utilize the charging delay afforded; to vary the charging time of such of time p ing is not adversely affected by such imperfec- 4 tions; and to so arrange the foregoing elements that they may be expeditiously incorporated in a control apparatus comprising another or other electron tube circuit or circuits.

A still further object is to maintain the condenser of a timing means in a normal or discharged condition whenever it is not being utilized. to effect a time delay; or in the event of an interruption of the supply voltage during the delay period, to normalize or discharge said condenser, to thereby suspend the operation of the timing circuit and restore same to normal condition.

Still another object'is to enable the timing deyice to function satisfactorily for effecting selected time delays of several minutes to a half hour, or longer, without being unduly sensitive to fluctuations in line voltage.

' Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawing, which, by way of illustration, shows a preferred embodiment and the principle thereof and what we now consider to be the best mode in which we have contemplated applying that principle. Other improvements of the invention embodying the same or equivadrawing.

In the operation of the timing device shown in v the drawing the switch SI is first closed to connect the'supply line wires Li and L2 to the wires LI and L8, respectively, and thereby condition the device for operation. However, a time delay period is not initiated until closure of the switch S2, which connects the wire L6 with the wire LI and also causes the control condenser C to commence building up a charge thereon. The amount of time which elapses from the instant this charge build-up of the condenser C is initiated until the charge potential reaches a predetermined critical value is determined by the setting of a variable resistor RI. The voltage across the condenser C influences the value of the potential at the control grid 26 of a grid-controlled gaseous discharge tube 2, which is preferably of the type known as a gas tetrode. When the grid potential reaches the value of the firing voltage of the tube 2, this tube is ignited and passes current through its anode circuit. A relay 3 is included in the anode circuit of the tube 2 and is operable, when the tube 1 has former T and a full-wave rectifier tube 4 have been provided, as well as a filter inductance FL and a filter condenser FC. The primary winding l or the transformer T is connected across the wires 1 -LI and L6. The secondary windings I, O and I are all provided with center taps. The secondary l5 5 supplies the voltage for the filament ll of the rectifier tube 1, and a conductor I leads from the center tapof the winding 5 to one terminal of the filter inductance FL, the other terminal of this inductance being connected to a conductor 9. The opposite ends of the secondary l are respectively connected to the anodes 4F and P of the tube 4, while a conductor II is connected to the center tap at the winding 6. The iilter condenser F0 is connected between the conductors 9 and II and cooperates with the series inductance F'Lto smooth out the pulsating potential furnished by the rectifier tube 4. Thus the conductors S and II aflord 'a substantially direct-current potential for the remainder of the apparatus to therlght 01 the condenser PC, as viewed in the drawing, the conductor 9 being positive with respect to the conductor II.

A fixed resistor R0, connected at one end to condoctor I, is -in series withthe variable resistor RI which in turn is in series with a fixed resistor R2 connected to conductor ll, so that the combined series resistance of the resistors RLRI and R2 is in parallel with the filter condenser E. A conductor I! is connected to the resistors RI and R2 at the junction thereof and to a conductor II, which latter conductor is connected to the center tap oi the secondary winding I and also to a conductor l3 which is connected to the cathode 1K oi the tube 2. Secondary I supplies the voltage tor the filament 2F 0! the tube 2.

The tube 2 includes a shield grid 2o" connected to the cathode 2K, and preferably a metal shield 28 is provided around the tube 2. The shield 28,

cathode 2K and shield grid 26' are preferably all connected to ground through the conductor 13. The anode 2? or the tube 2 is connected by a conductor II to one terminal oi the winding of the relay 3. The other terminal (if said winding is connected by a conductor II to one terminal of a normally open contact IBB ofv a relay 49, de

' Y scribed more fully hereinafter; and a conductor 1 II, tapped onto the conductor 8, is connected I the other terminal of the contact I98.

A resistor R3 and the control condenser C are 50 connected in series with each other and are also respectively connected to the conductors I and .l! in such a manner that the combined series impedance thus afforded is in parallel with the re- ,sistance comprising the resistors Rs, RI and m in series. The control grid 26 of the tube 2 is connected through a resistor R4 tothe resistor R8 and the condenser C at the junction thereof.

, Because of the connections which have thus been provided, the potential of the control grid 20 7" with respect to the cathode 2K is determined by I the voltage drop across the condenser 0 minus the 7 drop in voltage through the resistor R2, inasmuch .as the positive end 01' resistor Rlis connected to the cathode 2K through the conductors H, I!

and I3, while the positive terminal of denser C is connected to grid 20 through the re= sistor R4, the opposite or negative terminals of the resistor R2 and condenser C being connected together by the conductor iii.

One terminal of the condenser C is joined by a conductor I 5 to another conductor it which is connected to a terminal oi the normally open contact 33 of the relay 3. The other terminal of the condenser C is. connected through the conductor Hi to a conductor I? and thence through a resistor R5 to a conductor ll that leads to the other terminal of the contact 33. Hence when the relay I is energized, in a manner to be described hereinafter, the resistor R5 is shunted across the condenser C. This enables the condenser C'to discharge without sparking when the relay 3 is energized. The resistance 0'! R5 is negligible in comparison with the resistance of R3 so that condenser C is, in effect, short-circuited by theresistor R5 when the latter is shunted across it.

The conductors I8 and ii are also connected to the terminals of the normally closed contact HA of a relay it, the winding of which relay is connected across the line wires L3 and L5. If switch S2 is open, the wire L5 is not potentialized and therefore the relay I8 is deenergized, thereby connecting the conductor Ii to the conductor I! through its contact "A and maintaining the resistor R5 shunted across the condenser C. This prevents the presence of stray charges on the condenser C when the apparatus is not in operation. when the switches SI and 82 are closed to initiate operation of the apparatua'the relay i9 is energized. and opens its contact ISA and thereafter the condenser C will not be shunted by the resistor R5 unless and until the relay 3 becomes energized to close its contact 13, provided the line voltage does not fail and thereby deenergize relay ll, which would result in the closure of contact ISA 0! this relay and the 'reestablishment'oi' the shunt R5 across the terminals of the condenser C.

Before describingthe operation of the timing device shown in the drawing, it will be assumed that such device has been completely shut down,

which is to say, that both the switches Si and S2 are open. In this event, the first step preliminary to initiating operation of the device is to close the switch $9. This connects the wires L3 and L6 to the line wires LI and L2, respec tively, and thus potentializ'es the primary l of the transformer '1. The secondaries S and "I of transformer 'l thereupon supply current to the iilaments 4F and 2F of the rectifier tube 4 and control tube 2, respectively. Secondary 6 applies alternating potential to the anodes P and IP' oi the rectifier tube 4, and this tube thereupon furnishes a-pulsating full-wave voltage to and filter inductance FL and filter condenser FC. These elements function to smooth out the pillsatlng voltage and to apply a direct-current voltage on the conductors 5 and ill. the conductor 8 being positive with respect to the conductor to.

- Switch 82 is nowclosed to initiate operation of the timing device, and the delay period connmences to run as of this time. Closure-oi switch S2 connects wire L8 to wire L8, so that line voltace is now established across the wires L3 and L5. Relay 1% is thus energized to open its con iact HA and close its contact 393. Closure of contact I98 tentatively establishes the anode sir I cult of the control tube 2 by connecting conductors I and Zi-together, so that. there is a nection between conductors I6 and I8, inasmuch as contact 3B of relay 2 is open at the time operation of the timing device is initiated; therefore the resistor R5 is disconnected from the condenser C, it being recalled that resistor R5 is connected in parallel with the condenser C only so long as either of the relay contacts ISA or 3B is closed. When the shunt resistor R5 has thus been disconnected from the condenser C, the control grid 26 of the tube 2 is initially at a substantial negative potential with respect to the anode 2P because of the existence of a voltage drop in the resistor R3 and because there is no appreciable charge potential, as yet, on the condenser C, inasmuch as the equivalent imped-' ance or counter-voltage of a previously discharged condenser that is being newly charged is initially zero. Likewise, there is an initial negative potential on the grid 2G with respect to cathode 2K, the magnitude of which is to a large extent determined by the voltage across the resistor R2, it being recalled that such grid potential is dependent upon the voltage across the condenser C which is initially zero, minus the voltage drop through the resistor R2. As has been mentioned hereinabove the tube 2 is a gaseous discharge tube and is controlled primarily by the grid 26; therefore, there will be no discharge whatsoever between thecathode 2K and the anode 2P unless and until the potential of the grid 2G with respect to the cathode 2K becomes more positive, or less negative, as the casev may be, than a predetermined value, namely, the firing voltage of the tube;

Thus, at the initiation of operation of the timing device the control grid 2G of the tube 2 is at a negative potential with respect to the cathode 2K and also with respect to the anode 2P, and the initial negative potential of the grid 2G with respect to cathode 2K created by the voltage drop in resistor R2 is ordinarily suiiicient to block the flow of current through the tube 2. The magnitude of the voltage drop through the resistor R2 is initially dependent upon the setting of the variable resistor RI. II the resistor RI has been adjusted to a relatively low value, there is a relatively large voltage drop in the resistor R2 and the initial negative potential of the control grid 2G is comparativelly great; whereas it the resistance RI is at a high value, the drop in the resistor R2 is small and the initial negative potential of grid 26: is correspondingly small. As will appear subsequently, the initial magnitude of this grid potential determines the amount of time delay afforded.

Thus far it has been assumed that the countervoltage of the condenser C is relatively small, which is the condition that prevails initially upon removal of the shunt resistor R5 from around the condenser C when the switch S2 has been closed. However, the voltage across the condenser C commences to build up rapidly thereafter, this being due to the increasing counter-voltage oiiered by the condenser C to the iiow of current there-through. The condenser voltage tends to increase toward the upper limiting value of the diflerence in potential between the supply conductors 9 and III, and in the course of so doing. it rises to a level comparable with the value of the voltage drop through the resistor R2, so that the difference in potential between the control grid 26 and the cathode 2K or tube 2 gradually decreases. The magnitude of the potential on grid 2G, with respect to cathode 2K, thus tends to change gradually from a large negative value (voltage drop through R2) to a large positive value (voltage drop through R6 and RI). However, when this grid potential is decreased in value from its negative limit to a certain point, which in the illustrated embodiment is at or near zero voltage, where it can no longer prevent discharge from taking place between the cathode 2K and the anode 2P, the tube 2 commences to pass current, so that in addition to the current flow from conductor 9 through the resistors RI and RI to the resistor R2 and thence to conductor I0, there is another path 01 current flow from the conductor 9 through conductor 20, now closed relay contact I9B, conductor 2|, winding of relay 2, anode 2P, cathode 2K, conductors I3, I2 and II, and resistor R2 to conductor III, and this added current flow through the resistor R2 tends to cause the negative potential on the grid 2G, which up to this time had been decreasing, to tend to increase negatively. However, because of the characteristics of the tube 2,. any changes in the potential of the grid 2G, once discharge has been initiated, will have no appreciable eiiect on tube 2, so that the discharge continues until interrupted upon removal of the supply voltage, which ordinarily occurs upon the opening or either or both of the switches SI and .S2 for recycling or shutting down the control apparatus.

Thus, once the discharge has been initiated through the tube 2, it continues and the relay} therefore energizes and remains energized to maintain its contact 3A closed. The line wires LI and L2 will have therefore been connected to the line wires L3 and L4, respectively, by the time such discharge is established in the tube 2, the connection from L2 to L4 being effected through wire L6, closed switch S2, conductor L5, and the now closed relay contact 3A, and it being recalled that wire L3 was connected to line wire LI when switch SI was closed. Contact 3B of relay 3 closes and connects the resistor R5 across the terminals of the condenser C, and the charge which previously accumulated on the condenser C rapidly leaks of! through resistor R5.

In the foregoing description of operation it has been assumed that the timing device was originally in a shut-down condition, that is, with both switches SI and S2 open, in which event the switch SI was closed as a preliminary operation, and switch S2 was then closed to start the delay period running. However, the usual operation is that or recycling the apparatus. For example, assuming that an abnormal condition arises elsewhere in the system which requires that a new starting cycle be initiated, such as flame failure at one or more burners in a multiple fuel burner system having a common safety control means governed by the timing device in the drawing. the switch S2 is first opened to thereby deenergize relay I5. Opening of relay contact [98 upon deenergization of relay I9 opens the anode circuit of the tube 2, extinguishing the discharge therein and deenergizing relay 3. Opening of contact IA of relay 3 removes potential from the line wire L4. Opening of contact 3B of this relay tends to remove the shunt resistor R! from the condenser C; however, closure of contact ISA of relay is aforesaid.

The time interval which elapses from the indenser o, and the anode 2? of the tube 2 passes current to cathode 2K, the winding of relay 8, in series with the anode 2?, is thereby energized to cause relay 2 to close its contacts in and 33., Closure of contact 2A connects the wires L4 and L5, so that line wire L2 communicates with the 1 wire L4 through wires L6 and L6. Closure of. contact 33 connects the resistor R in shunt with the condenser C, for discharging the latter.

stant the switch S2 is closed to the instant when ent upon two factors, namely, the initial voltage drop through the resistor R2, and the rapidity with which the condenser C commences to build up its full charge potential after the shunt. R5

has been removed therefrom. The rapidity of charging of the. ,eondenser C, in turn, is deter- -the relay 3 becomes energized is mainly dependanode circuit of the tube 2 remains energized, so

longlas switch 82 remains closed to maintain mined largely by the values of the resistance R3 and the capacity of the condenser C, although-it is to some extent influenced by the values of the resistors R8, RI and R2, and other impedances' in the timing delay circuit. The values of resistance and capacity of the resistor R3 and condenser C, respectively, as well as the values of other circuit impedapces are so selected as to accommodate the maximum amount of time delay that may be required inlpractice. For example, where the'maximum delay is of relatively short duration, say, up to about five minutes, the capacitance of condenser C can be made lower in relation to the resistance of the resistor- R2 than would be the case if the contemplated time delays required were on the order of, say, ten to twenty minutes.

Assuming that the values of R2 and Chave been selected to provide the desired initial rate of build-up of the voltage across the condenser C, for accommodating a given range'of delay intervals, the next consideration is the selection of available range, andthisis accomplished by adlusting the resistor RI. For example, if a relatively long delay interval is to be selected. the resistance of R! is made comparatively low to a particular amount of time delay within the This disrupts the relation which existed between the potentiais on grid 2G and cathode 2K at the initiation of the discharge-in tube 2. However, as has been explained previously, once the tube 2 has been set in operation, it continues to function independently of any subsequent changes. in bias of the grid 26, and hence relay 3 in the contact NE of relay I! closed, and provided the rectifier tube 4 continues to potentialize the conductors- 9 and l0, which jisto say, so long as switch Si is closed .to supply power to the transformer T; and also providedthe supply voltage doesnot fail.

If arelatively shorttime delay is required, the

resistor BI is adjusted to a high value to thereby decrease the voltage drop in the resistor R2. This has the eifect of decreasing the diilerence in potential initially existing between the upper or positive terminalsof the resistor R2 and the condenser C, as viewed in the drawing, so the. the potential of the control grid 2G is less negative with respect to the cathode 2K, on starting, than was the case in the preceding example of operation. Hence, the condenser C does nothave to build its voltage up to such a high value as formerly in order to equalize the potentials on the cathode and control grid of the tube 2, and thus a shorter time delay is afforded between the initiation of operation of the device and the instant when the tube 2 commences to discharge.

As has been stated hereinabove, the shield grid 2G of the tube 2 is preferably connected di- 1 rectly to the cathode 2K'of this tube. Under provide a high voltage drop through the fixed resistor R2 connected'in series therewith. Under these circrimstanceathe voltage that is built up across the condenser 0 upon closure of the switch S2 and removal of the shunt R5 will'not approach the value of the voltage drop in the resistor R2 until a relatively long time interval has elapsed. Therefore, the potential on the these. conditions the firing voltage of the tube 2, is very close to zero, in'comparison with the high negative potential originally impressed on the control grid 2G at the initiation of the timing period. It has been found that under these conditions the length of the timing period is substantially independent of fluctuations in line voltage, provided such fluctuations do not decathode 2K of the tube 2, which cathode, as has I been explained, is connected to the upper or' positive end of the fixed resistor R2, as viewed inthe drawing, remain appreciably more positive than the potential on the control grid 2G of said tube for a relatively long period, inas-" much as the potential of grid 2G is dependent upon the voltage drop existing across the con- N denser C: or in other words, a sufiicient negative potential is present on the grid 2G. to prevent discharge between cathode and anode of the tube 2 so long as the voltage drop across the condenser C remains substantially below the level of thevoltage drop in the resistor R2.

7 Discharge in the tube 2 is not initiated until 2 these voltages approach equality, it being assumed, for the reasons given hereinafter, that the firing voltage is at or near zero; but when such discharge occurs, upon equalization of the voltage drops across the resistor R'laud convelopiinto sudden and prolonged changes. To render the circuit more'highly accurate under the latter circumstances, it will be desirable to bring the firing voltage of the tube 2 exactly to zero, and this can be conveniently done by aflording an adjustable bias on the shield grid 26', such bias having a range of adjustment, say," from zero to three volts; By proper selection of the shield grid bias the firing voltagecan be 'brough'ttozero, and this renders'the firing time substantailiy independent of the magnitude of the current flowing through the resistors R8,

RI and R2 at the criticaiinstant when the potentiai on the control grid 2G is approaching the firing voltage. 'Hence, the device 'will not be unduly affected by sudden variations in the supply voltage occurring at this critical time in the delay period. However, under ordinary' circumstances, the fluctuations in voltage are not'severe enough to necessitate; adjusting the firing voltage of the tube 2 exactly to zero, and

therefore the preferred arrangement shown in the drawing is highly satisfactory.

gree of selectivity of time delay intervals on the order of, say, a minute to a half hour, or longer, is available by merely adjusting the resistor RI. Preferably, in a commercial installation, the adjusting means is kept locked to prevent manipulation thereof by those having authority to determine the amount of time delay to be afforded. Moreover, there ,is no excessive heating 0! the elements of the device, so that they may becompactly assembled.

any persons other than Another advantage is that the construction is 5 distinctly electrical in nature, rather than mechanical or electro-mechanical,does not employ moving parts, and is peculiarly adapted to be assembled and serviced as part of other control apparatus of a similar character.

Another feature is that the tube 2 is rendered operative in dependenceupon the charging time of the condenser C, rather than the discharge time or the condenser; hence, it the condenser has excessive leakage and cannot build up a proper charge, the negative potential of the grid 2G of tube 2 remains at a value where it is able to block current flow through the tube 2 and thereby prevent the relay 8 irom energizing to initiate operation of the apparatus controlled thereby. In electrical timing devices which are controlled by the discharge of a condenser, excess leakage is not only fatal to the accuracy of timing, but may prematurely initiate operation It is to be noted that the adjustment oi the timing device to aflford a selected delay interval is eiiected solely by manipulation of the variable resistor RI and that it is not necessary to alter the capacity of the condenser C ior such purpose, or to add or remove condenser units in case a plurality oi condensers is utilized. The capacity or condenser C is preferably madesufliciently high so that the contemplated range oi potentials to charged to equalize the voltage drops that may be initially set up in the resistor R2 by adjustment of resistor RI, will lie well within the substantialiy straight-lineportions or the charging shunt resistor R5 across the condenser C. If the power supply should fail while the apparatus is being recycled and before the tube 1 has fired, the relay is deenergizes and closes its contact MA to thereby-discharge thecondenser C through the resistor R5 in preparation for a subsequent recycling operation.

A fluctuation in supply voltage occurring while the condenser C is charging does not adversely affect the accuracy of the device, but merely produces a deviation in the charging characteristic,

' the effect of which, as has been noted hereinabove, is minimized by the peculiar circuit arrangement in so far as tube 2 is concerned.

Thus while we have illustrated and described a selected embodiment of our invention it is to be understood that it is capable of variation and modification and we therefore do not wish to be limited to the precise details set forth but desire to avail ourselves 0! such changes and alterna tions as fall within the purview of the following claims.

I claim: I

1. In a timing control device for selectively determining the amount of time delay aflorded subsequent to the initiation of operation thereof and prior to the operation of other apparatus to be controlled thereby, the combination, with a direct-current voltage supply means, of, a resistance having at least a portion thereof variable, an impedance including a condenser, direct-current supply conductors connecting said resistance and said impedance in parallel to said directcurrent voltage supply means, a gaseous discharge of the apparatus controlled thereby. 3;

tube including a cathode, a control grid and an anode, means connecting said cathode to a point in said resistance the potential of which is intermediate the potentials of said supply conductors and the magnitude of which potential is de- 40 termined by the setting of said variable portion of said resistance, other means connecting said which the condenser must be characteristic (or, variation of potential with'BO respect to time) 01 the condenser C. This minimizes the harmful effect of such deviations in the charging characteristic of the condenser as may be encountered in practice, ior manifestly if the voltage drops that may be initially set up' in the resistor R2 are, in all instanceawell below the level at which the characteristic of the condenser C becomes appreciably asymptotic with respect to its steady-state value, a slight deviation in the characteristic oi the condenser pro- 00 its contact 33 and places the shunt resistor R5 across the condenser C for discharging the same without sparking.

When the apparatus is in its normal inoperative condition, relay i9 is deenergized and by control grid to a point in said impedance the potential of which isof a magnitude that varies in accordance with the state of charge of said condenser, whereby the potential of said control grid with respect to said cathode at any instant is dependent upon the difference between the value c; the. voltage drop through said condenser at that instant and the value of the voltage drop through a certain portion of said resistance, shunt means normally connected in parallel with said condenser, a starting relay operable to disconnect said shunt means from said condenser to thereby. render said direct-voltage supply means effective to initiate charging and consequent voltage build-up of said condenser and thus initiate operation of the timing control device and operating in the event 0! failure of said voltage supply means to re-connect said shunt means to said condenser, means connecting said anode in a circuit to said voltage source and including an operating relay which is rendered operative to control said other apparatus when the voltage drop through said condenser is oi a value suflb.

cient to bring the potential on said control grid with respect to said cathode to a predetermined firing voltage for igniting said tube, the time interval elapsing between such initiation of operation of the timing control device and the instant when the voltage drop through said condenser has reached said suflicient value being determined by the setting of said variable portionoi said resistance, andmeans under control of said operating relay ior re connecting said means of its closed contact us maintains the shunt means to said condenser when said opersupply conductors lnparallel withthe first-named resistance, a grid-controlled gaseous discharge termined by the voltage drop-through said .con-

denser less the voltage drop through a portion of ating relay has been operated, said starting relay and said operating relay thereby functioning to shunt said condenser for discharging the same at all times except while the time delay operation is being duly performed by the timingdevice.

' 2. In-an electrically operated time delay device, a source of direct-current voltage including two supply conductors across which a predetermined direct-current voltage is maintained, a resistance connected between said supply conduc tors, an impedance comprising a condenser and another resistance connected in'series with each other, said impedance being connected to said tube including a control grid,'a cathode and an anode, means connecting said anode in-a circuit to the positive side of said voltage source and including operating means. which is rendered eflective when said tube is ignited to initiate operation of other apparatus governed by the timing device, means connecting said cathode to a point in the first-named resistance the potential of which" is intermediate thepotentials of said supply conductors, means connecting said control grid to said'condenser at the junction thereof with said other resistance, whereby the potential said control grid with respect to said cathode is dethe first-named resistance, and means for ran-- dering said direct-current voltage source effective to initiate charging of said condenser to thereby cause said condenser to build up a volanode circuit for operating said operating means when the voltage of said condenser becomes sub- 7 means controlled by said operating means for discharging said condenser when said tube is ignited. r

5. In a circuit of the character described, the

5 combination of a condenser. adapted to be energized from a voltage supply line, means including a device responsiv'efo a'predetermined difference in potential across-said condenser for energizing a control circuit, meanaunder the control 01 said energizing means 'ior automatically discharging said condenser, and means responsive to a failure in the voltage supply line for automatically rendering said condenser discharging means eiiective to discharge said condenser.

combination or a condenser adapted to be ener 1 gized from a voltage supply line, means including a device responsiveio a predetermined difference in potential across said condenser'ior energizing a control circuit, means underithe control oi'said energizing means for automatically discharging said condenser. and means including a relay responsive to a'tailure in the voltage supply line (or automatically rendering said con-' 5- denser discharging means eflective to discharge said condenser.

7. In a. circuit of the character described. the combination or a condenser adapted'to be ener gized from a voltage supply line, means including a device responsive to a predetermined diflerence in potential across said condenser for ener- 'gizing a control circuit, a'resistance under the control of said energizing means for automatically discharging said condenser. and means responsive to a failure in thevoltage supply line T for automatically rendering said resistance eiIective to discharge said condenser. a 18. In a circuit 0'! the character described, the

combination of a condenser adapted to be energized from a voltage supply line. means including a device responsive to a predetermined difference in potential acrosssaid condenser Ior enercontrol of said energizing means for automatistantially equal to the voltage drop through said cally discharging said condenser, and means inportion of the first-named resistance.

3. In an apparatus as set forth in claim 2, means responding to-a failure of said voltage source to discharge said condenser.

' 4. In an apparatus as set forth in claim 2;

eluding a relay responsive to a failure in the voltage supply line tor automatically rendering said resistance eitective to discharge said condenser.

. DONALD E. RICHARDSON. ROBERT E. YATES.

8. In a circuit of the character described, the 

